A major market for thermoplastic polyolefins (TPOs) is in the manufacture of automotive parts, especially exterior parts like bumper fascia and body side-molding and interior parts like instrument panels, door trim panels and side pillars. These parts, which have demanding stiffness, toughness, scratch resistance, and, in some cases, uniform surface appearance requirements, are generally made using an injection molding process. To increase efficiency and reduce costs, manufacturers have sought to decrease melt viscosity, decrease molding times, and reduce wall thickness in the molds, primarily by turning to high melt flow rate (MFR) polypropylenes (MFR greater than about 35 dg/min). However, these high MFR polypropylenes tend to be low in molecular weight, and therefore difficult to toughen, resulting in low impact strength especially at sub-ambient temperatures. To achieve a satisfactory balance of stiffness, toughness, and processability, one option is to combine a moderate MFR polypropylene, a high content of polyolefin modifier (typically ethylene-propylene rubber and/or plastomer) and a reinforcing filler. Unfortunately, this approach has limitations in terms of the maximum MFR that can be achieved while still meeting the stiffness and toughness requirements. In addition, it can lead to poor surface appearance, in terms of the appearance of flow marks (or “tiger stripes”) or other surface defects.
Furthermore, the problem in the use of elastomers to improve the physical properties of TPO's is twofold. First, the compositions lose strength because of the elastomer and second, the elastomer contributes to a softer surface, which is thus more easily scratchable. Industrial and automotive applications frequently use filled polymer systems to provide desirable mechanical properties, such as stiffness or scratch/mar resistance. However, use of polymer fillers sometimes adversely affects the polymer's surface smoothness and can cause deleterious effects on the appearance of scratches or mars in the polymer systems. An example is the white color of a surface scratch often exhibited by a talc-filled polymer system. Thus, the usefulness of TPO is limited for many applications in the automotive industry and elsewhere where low temperature requirements and other physical properties such as scratch resistance and material shrinkage control require use of impact modifiers and other additives.
In the art, polyvinyl chloride is a material that has been used where softness (feel) of the finished surface and good processability are desired. This is a particular need in the automotive industry, where attractive surface properties, as well as hardness and scratch resistance of the material are desired, while the material should be quick, easy and cost-efficient to process. Polyvinyl chloride is, however, not recyclable. Therefore, there is still a need in the art to provide recyclable materials that can be used as alternatives to polyvinyl chloride for the fabrication of articles such as rubbery, thinner sheets used as skin layers over a core substrate, for use in automotive interior parts, such as instrument or door panels. Such materials should also have a good processability, i.e., should exhibit high flow under high shear conditions such as during injection molding for a quick, easy, and cost effective production. Furthermore, the finished article made from such materials should have attractive surface properties, in particular should have a soft feel, without feeling sticky (or non-sticky after heat aging), and should not exhibit any visible gloss change after heat aging. Additionally, the article should have good tear resistance, and its surface should be scratch resistant.
Even though TPO's are penetrating into the skin market, they generally suffer from poor grain retention; poor scratch/mar resistance; high cost of priming and painting; a limited processing window for calendaring and thermoforming compared to PVC; low tear strength; and/or poor drawability (important in thermoforming). The TPOs that meet the skin criteria are generally highly tailored and/or compounded, most of them coated, and therefore are not as cost-effective as one would like. However, TPO economics, expanding property profiles, good recyclability, and the potential for parts integration through injection molding continue to make them the most desirable materials to build on.
Thermoplastic vulcanizates (TPV) also hold promise, since there is a belief that cross-linked rubber particles could impart favorable grain retention and scratch resistance along with other beneficial properties. However, most of the TPVs made today contain large amounts of oil, which contribute to fogging and volatiles, and are more expensive than TPOs. Furthermore, most of TPV's have low melt flow rates and are difficult to process.
Other references of interest include: US 2006/0281868, US 2008/0027173, US 2008/0033124, WO 2003/040201, US 2004/0054100, U.S. Pat. No. 6,319,998, U.S. Pat. No. 6,284,833, U.S. Pat. No. 6,512,019, U.S. Pat. No. 7,365,136, U.S. Pat. No. 6,441,111, U.S. Pat. No. 6,806,316, U.S. Pat. No. 5,962,595, and EP-749992.
According to the present invention there is provided a polymer blend which exhibiting a unique combination of a high melt flow rate combined with high tensile strength, tear strength and elongation at break, is attractive for injection molding applications and particularly for injection molding components having a scratch resistant skin.